Variety Of Mutation Rate Among Species

An early and unexpected discovery from the molecular evolution was the molecular clock - a relatively constant rate of molecular evolution proposed according to the neutral theory of Kimura by Zuckerkandl and Pauling in the 1960s. This theory thought that if two species origin from the same ancestor, we can find the homological molecular from their common ancestor between them. Many factors can affect the precision of the clock such as population size, generation, volume, mutation rate etc and mutation rate is the essence. Mutation rate plays a pivotal role in evolution genetics, which varies greatly among species. However, no one has concentrated on the mechanism which results in the diversity of spontaneous mutation rates among species.The molecular evolution is the result of interaction between nature selection and random genetic drift. We consider that the diversity in any character is formed by nature selection in some extent. Similarly, the mutation rate could be moved by nature selection because of the nucleotide mutations include a few classes.We investigate the likely microbe mechanisms by establishing a model providing kinds of initial conditions such as population size, genome size, the distribution of fitness effects of new nucleotide mutations, fitness of initial population, mutation type etc, in which a group of microbes composed of ten subpopulations is revolved under the nature selection simulated by computers. In the model, subpopulations are initiated with a series of different mutation rates. Maintain the total of population constantly through selection and random genetic drift for hundreds of generations. As a result, only few subpopulations still survive so that a distribution of mutation rate could be obtained by probabilities of every subpopulations fixed. The results suggest that under a gamma distribution of mutation effect the mutation rate per base pair could be appropriated well by a normal distribution.Furthermore, some other parameters including: 1) genome size, 2) proportion of mutation classes and 3) fecundity are changed respectively or simultaneously to study their impacts to the final fixation of mutation rate in the group. The simulation results show that under same condition there is a negative linear correlation on a logarithmic scale between the optimal mutation rate per base pair and the mutation rate per genome size maintain nearly an invariant. Moreover, the rate varies about 1.5-fold and 2-fold respectively as the shape parameterαincreases from 0.2 to 2 when fitness effects of new mutations followed the gamma distribution and the fecundity changes from 2 to 3.We hope that these results can help to revise the molecular clock.